S N

N S

113

OUR PLACE IN THE UNIVERSE Geological Processes

Questions1. What is the difference between a constructive and destructive plate boundary?2. Explain why the majority of earthquakes and volcanoes occur near plate boundaries.3. Give three pieces of evidence mentioned above in support of the idea of plate tectonics.4. Why did people find it difficult to accept Wegener’s ideas?5. What is the name of the process that causes the material in the mantle to circulate and drag the plates

along?6. Describe how the magnetization of the rocks of the oceanic crust could be used to show that the ocean is

growing wider over millions of years.7. Describe and explain the differences between the collision of two continental plates compared to a

continental and an oceanic plate.

Theory of plate tectonics

Magma rises and pushesplates apart making the

ocean grow wider.

The Earth’s crust is made up of anumber of independent plates.

Constructiveplate boundary Plates can move

relative to each other.

Newmountainsare pushed up by collision betweentwo continental plates.

Trench

Earthquakesoccur as plates

stick as theyslide past each

other.Mid ocean

ridge

Tectonic plates are draggedalong by the mantle as itconvects under them.

Radioactive decay ofisotopes in the mantle

provides the heat.

Convectioncurrents pulling

plates alongDestructiveplate

boundary

Mantle – very hot and under pressure.Although effectively solid, it can flow very slowly byconvection due to the very hot core of the Earth.

Rocks either side of oceansoften match, suggestingthat they were once joined. ‘Fit’ of continents, e.g. Africa

and South America suggeststhat they were once joined.

Without mountainbuilding, mountainswould eventually beeroded to sea level.

Largecontinent

broken apart.

Wegener’s theory was originally not accepted. He did not have any

convincing reasons explaining how the plates moved. He overestimated the rate atwhich plates would move; the rate did not

agree with observations. Magma

Magma

The Earth’s magnetic field reverses every fewmillion years; the North Pole becomes the

South and vice versa.

New rock ismagnetized by

the Earth’smagnetic fieldas it solidifies.

New rock formed after a reversal of thefield will have the opposite magnetization.

There were simpler explanations of the same evidence.

Magnetic striping of rocks is evidencethat the oceanic plates are moving apart.

Subduction zone

Oceanic ridge slides under thecontinental plate and melts. Someof this molten rock (magma) rises

to the surface via cracks in thecrust forming volcanoes.

Platestypically moveat about 10 cm

per year.

Similar fossils occur in nowseparatedcontinents,suggesting theywere joined whenthose animals andplants were living.

Africa

SouthAmerica

Atl

anti

c oc

ean

Oceanic plateOceanic plate

Continentalplate

Continental plateOn

esli

des under Plates collide.an

other.

Suggested byWegener (1912)

Evidence forplate

tectonics

114

OUR PLACE IN THE UNIVERSE The Solar System

Questions1. Which of the following orbit the Sun directly and which orbit planets?

Comets, moons, asteroids, artificial satellites, planets.2. Explain why the density of Jupiter, Saturn, Uranus, and Neptune is a lot less than that of Mercury, Venus,

Earth, and Mars.3. Using the data in the table show that: a. The circumference of the Earth’s orbit is 942 million km.

b. The time the Earth takes to orbit the Sun is 31.6 × 106 s. c. That 31.6 × 106 s = 1 year.4. Plot a graph of surface temperature vs. distance from the Sun. State and explain any trend you see. One

planet is anomalous, which is it and give a scientific explanation for why it does not fit the trend?5. Explain why the speed of a comet decreases as it moves away from the Sun.

Oort cloud – objects made of ice and dust orbiting the Sun far beyond Pluto. Sometimes thesefall in towards the Sun and become a comet due to minor gravitational disturbances.

Quantity Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Mean distancefrom sun (orbitradius),million km 57.9 108 150 228 778 1430 2870 4500 5900

Time to orbit the Sun, years 0.241 0.615 1.00 1.88 11.9 29.5 84.0 165 248

Orbital speed, km/s 47.9 35.0 29.8 24.1 13.1 9.64 6.81 5.43 4.74

Equatorialdiameter, km 4880 12 100 12 800 6790 143 000 120 000 51 800 49 500 ?3000

Mass(Earth = 1) 0.0558 0.815 1.000 0.107 318 95.1 14.5 17.2 ?0.010

Density g/cm3 5.600 5.200 5.520 3.950 1.310 0.704 1.210 1.670 ?

Moons 0 0 1 2 16 17 15 2 1

Typical surfacetemperature, ºC 167 457 14 –55 –153 –185 –214 –225 –236

Atmosphere None Carbon Nitrogen, Carbon Hydrogen, Hydrogen, Hydrogen, Hydrogen, Nonedioxide oxygen dioxide helium helium helium helium

TheSun

Comet speed increasing (strength of Sun’sgravitational attraction increases, comet

transfers GPE to KE as it approaches the Sun.)

Asteroids – orbit between Jupiter and Mars.They are thought to be some rocky debrisleft over from the formation of the solarsystem, which Jupiter’s strong gravityprevents forming into a planet.

Orbit 1 year

Thick atmosphere of carbondioxide and sulphuric acid traps solar radiation in a runawaygreenhouseeffect.

Decreasing temperature as th

e intensity of so

lar radiation

decreases according to th

e inverse sq

uare law (see p31).

Pluto

Neptune

Uranus

Saturn

Jupiter

Mars

Earth

VenusMercury

Gas giants

Rocky planets

Comets,made from iceand dust, havehighly elliptical

orbits.

Moons orbit the planets(they are natural satellites).

A few asteroids have orbits,which cross the orbit of Earth.

The planets orbit the Sun in elliptical (slightly squashed circle) orbits. The Sun is at one focus of the ellipse.

Comet speed

decreasing

115

Questions 1. Make a list of the advantages and disadvantages of space telescopes compared to ground based

telescopes.2. Why do you think optical telescopes that collect visible light are often placed on mountains whilst radio

telescopes can be at sea level?3. Will the image in a refracting telescope be upright or inverted? Use a ray diagram to illustrate your

answer. Suggest two advantages of having a very large objective lens and explain why there is a limit onhow big the objective lens can be.

4. The aperture of a reflecting telescope is 0.7 m in diameter and it collects light of wavelength of about0.00000055 m. Its objective mirror has a focal length of 0.4 m and its eyepiece a focal length of 1.50 cm.The diameter of the Jodrell Bank radio telescope dish is 76.2 m and the wavelengths it collects arearound 1 m. a. What is the angular magnification of the reflecting telescope?b. Which telescope would you expect to suffer the most from diffraction?

5. Suggest at least three reasons why astronomers need to work together in international groups.

OUR PLACE IN THE UNIVERSE Telescopes and Types of Radiation Used to Learn About the UniverseEverything we know about space outside the solar system comes from analyzing the electromagnetic radiationcollected from space by telescopes.Different objects in space emit different wavelengths.

Coolnebula Warm

nebula

Infrared

Stars

Theradiationfrom manyobjects inthe Universeis very lowintensity.Telescopesneed to belarge tocollect asmuch aspossible.

The atmosphereblocks some

wavelengths sosatellites are used

to detect these.E.g.

the Kecktelescope in

Hawaii

Ultraviolet

Black holes and centres of galaxiesX-raysorgammarays

Space telescopes(e.g. Hubble)

Expensive to place in orbit and is difficult andexpensive to maintain.

Radio telescope,

e.g. Jodrell Bank in

Cheshire, UK

Electromagnetic rays arediffracted by an aperture – see p39.

The bigger the aperture theless diffraction and the better the quality of theimage.

As radiowaves are so long, radio telescopes need to have very bigreflecting dishes to avoid too much diffraction.

The angular magnification of a telescope is given by

Angular magnification = focal length of objective lensfocal length of eyepiece lens

Lens

Eye

All large telescopesuse mirrors as theycan be supportedacross their whole

diameter (up toabout 6 m).

A lens this sizewould distortunder its own

weight and ruinthe image.

Mirror(objective)

Plane mirror

45°Light from distantstars (parallel)

Simplerefracting telescope

Telescope on amountaintop

Above clouds

Away from cities whichproduce light pollution

Less atmosphere to lookthrough (atmosphere containsdust and slightly refracts light

reducing the quality of the image).

Large reflecting telescope. (These give better quality images than

refracting telescopes.)

Light from stars (parallel rays)

Large objective lens to collect asmuch light as possible

Longfocal length –

weak lensVirtual image appears at infinity

Short focallength (powerful)

EyepiecelensReal inverted image

Radiowaves

Visible light Radiationdoes not

have to passthrough theatmosphereto reach it.

N

W

E

West

East

Moon

116

OUR PLACE IN THE UNIVERSE The Motion of Objects in the Sky

The Moon orbits the Earth once every 27.3 days. It also takes27.3 days to rotate once so it always presents the same side to

the Earth.

The phases of the Moon take about 29 days to complete a cyclebecause the Earth has changed its position during the cycle as it

orbits the Sun.

Not to scale

Questions1. Do most objects in the sky appear to move east to west or west to east? Which objects do not always

follow this pattern?2. What is the difference between a solar day and a sidereal day?3. What is the difference between a lunar eclipse and a solar eclipse?4. Why do we not have an eclipse once a month?5. Why does the Moon appear at slightly different places in the sky each night at the same time?

Solar eclipse

Moon’s shadow falls on the Earth

Horizon – looking north

The Earth orbits

the Sun in one year =

365.25 days

Lastquarter

In one siderealday, the Earth

moves 360º/365.25= 0.986º around

the Sun.

New Moon

Waningcrescent

Waxingcrescent

Firstquarter

GibbousMoon

FullMoon

GibbousMoon

5°

Must be a new Moon

The Earth spins on its axis, through360º once every 23 hrs 56 minutes.

This is asidereal

day.

Distant starsThe Moon’s orbitis tilted at 5º tothe Earth’s orbit

During one sidereal day, theMoon completes 1/27th of anorbit. For the Moon toappear in the same place inthe sky the Earth will haveto turn 1/27th of a revolutionfurther. This takes about anextra 49 minutes. Therefore,moonrise occurs once every24 hrs 49 minutes.

Lunar eclipseMoon in Earth’s shadowEclipses do not happen once each month

because the Moon’s orbit is tilted so isusually above or below the direct line

between the Earth and Sun.Some planets are visible with thenaked eye (Mercury, Venus, Mars,Jupiter, and Saturn). They follow

complicated paths in the skybecause they too are moving

around the Sun, at different speedsrelative to the Earth. The termplanet means ‘wandering star’.

The sun appears to take(23 hrs 56 minutes ×

0.986º)/360º = 4 minuteslonger to move from east to

west across the sky and backto its starting point. This

makes a solar day 4 minuteslonger (i.e. 24 hrs exactly)

than a sidereal day.

Summer Winter

Sun

DayDayAt night, starsin this directionare seen.

Different stars are seenin different seasons.At night,

stars in thisdirectionare seen.

Sun

Lookingup the stars

appear to move from theeast to west across the

sky, returning to thesame position once

every 24 hrs 56minutes when the

Earth hascompleted a

revolutionabout its

ownaxis.

Must be afull moon

EARTH

OUR PLACE IN THE UNIVERSE Exploring Space

117

Questions 1. What are the three main ways scientists find out about the Universe?2. Copy and complete the following table to summarize the advantages and disadvantages of two ways of

exploring the solar system.

3. It is proposed to send astronauts to Mars. Apart from the journey time of a couple of years, what otherconsiderations are necessary when designing a spacecraft to make the journey?

4. ‘Exploring Space is a waste of money that would be better spent on giving aid to people who live in poverty.’ Do youagree or disagree with this statement? Give some explanation to try to convince somebody to support your view.

5. Explain why manned missions outside the solar system are very unlikely.

The Universe is vast. The light from the nearest star takes 4.2 years to reach Earth. If we could build aspaceship to travel at the speed of light the round trip journey time would be as follows.

Manned spaceflight Unmanned robotic probes

Advantages

Disadvantages

8.4 years

Neareststar – 4.2light years

52 000 years

4 million yearsCentre of ourgalaxy 26 000light years

Nearest galaxy –Andromeda –

2 million light years

PilotingSpacecraft

maintenance

So how do w

e go about

Probes can

Spacecraft gains momentum and hencekinetic energy as the gravitational pull

of the planet acts on it. Planet losesmomentum and slows down by an

immeasurably small amount due to itshuge mass.

Probes can withstandconditions lethal to humans(large temperature ranges

and high levels of radiation).

Lighter.Less fuelneeded.Moreeconomical.

By unmanned robotic

spacecraft (probes).Only practicalwithin the solarsystem.TheVoyager II probe took 11 years toreach Neptune.

Send back data on magneticfield, radiation, andgravitational field.

Flypast

planets

Shieldingneeded

Exposure to radiationfrom the Sun and

cosmic rays

Send backdata on

atmosphereand

temperature.

Dig up soil samples (e.g. NASA’s Vikingand Opportunity probes to Mars), carry out a chemical analysisand send back the results.

Sendback

images

Orbitplanets

Food?

Fuel?Take gym

equipment.

Prolonged weightlessness causes muscle loss andheart deterioration, as they do not continually have

to work against gravity as they do on Earth.

Water?

Air?

Astronautshave been

to the Moon.A journey to Mars might be

practical – it would take a fewyears for a round trip.

Air and water needs to berecycled. Carbon dioxide from

the astronaut’s respirationneeds to be removed.

Heatingneeded –

it gets verycold in spaceespecially as

the spacecraftmoves away

from the Sun.

Be lost ifthey

malfunction.

Fuel

can

besa

ved

byus

ing

grav

itatio

nalf

ield

sof

plan

ets

or

the

Sun

toac

cele

rate

prob

es.

No

humans on

boar

d soles

s risk

By manned spaceflight.

New

technologies

Astronauts

canm

akedecisions

and

Collecting informationabout the Universe?

(see p115, 119, 121, 125, 126).

No food, air or

water needed

Land

aredeveloped

Remote piloting of probes is hard

due to time delay for signals to arrive (4.2 hours toNeptune!). Astronauts couldmake immediate corrections.

E.g. Gore-Tex waterproofbreathable material was first

designed to make spacesuits.

deal with

problems

asth

eyarise.

Fro

mth

eel

ectr

omag

netic

radi

atio

nre

ceiv

edon

Earth

118

m M

Rr

F

f

Earth

F

a

Questions1. A planet orbits the Sun. What would happen to the size of its gravitational attraction to the Sun if:

a. It doubled in mass but stayed in the same orbit?b. It stayed the same mass but moved to an orbit twice the distance from the Sun?

2. What happens to the orbital period of a planet as you move away from the Sun? Does the table on p114confirm this? Give two reasons why the orbital period varies in this way.

3. A geostationary satellite has a mass of 5 kg and an orbit radius of 42 × 106 m.a. Show that its orbit circumference is about 260 × 106 m.b. Given that its orbital period is 86 400 s, show that its orbital speed is about 3000 m/s.c. Therefore, show that the centripetal acceleration is about 0.2 m/s2.d. Explain why the satellite’s weight in this orbit is about 1 N.

All masses exert gravitational attractions on all other masses.

OUR PLACE IN THE UNIVERSE Forces in the Solar System

Equal

Pull ofM on m

Pull ofm on M

r

v

v

vv

v

F

F F

F

F

SUN

This gravitational attraction is:• Proportional to the product of the two masses (F ∝ M × m)• Inversely proportional to the square of the distance between

their centres of mass (F ∝ 1/r2).

F

F/4F/9

r 2r 3r 4r Separation

The planets follow nearlycircular paths around the Sun(slightly elliptical). To do sothey need centripetal forceacting towards the centre oftheir orbit, towards the Sun.

Mathematics

Orbital circumference = 2πr

Orbital period = T

Therefore orbital speed, v = 2πr/T

Orbiting body

Centripetal force provided bygravitational attraction is the netresultant force.

As force and distance travelled are always perpendicular,no work is done (N.B. remember work = force × distancein the direction of the force). Therefore, the body does notneed any energy to be transferred to stay in orbit.

Velocity of object at 90º(tangential) toacceleration.

Gravitational attraction to Earth

Accelerationdue to freefall

F = m × (v2/r)

Equal

W = m × gWeight = mass × gravitational field strength

Is ca

lled

Prov

ides

This centripetal force is provided by thegravitational attraction between the planetand the Sun.

Satellites and moons orbiting a planet alsoneed centripetal force acting towards theplanet at the centre of their orbit. It is provided bythe gravitational attraction between the satellite ormoon and the planet.

From p19 Centripetal force = mass × velocity2/radiusof orbit.

Therefore, to stay in orbit at a particular distancefrom a larger body, a smaller body must travel at aparticular speed in order that the centripetal force

required is exactly provided by the available gravitational attraction between the

masses.

Advanced maths tells us that thelarger the orbit radius the slowerthe body must move because of

the weaker gravitationalattraction. In addition, the orbitcircumference is bigger so the

orbital period rapidly increases.

Time for oneorbit = orbitalperiod.

If there were no gravitationalforces, the planets and moonswould continue to move in a

straight line according toNewton’s First Law, as therewould be no forces acting on

them. They would drift off intospace.

Centripetalacceleration

Centripetal force = mass × centripetal

acceleration

119

OUR PLACE IN THE UNIVERSE The Structure of the Universe

Questions1. List the following objects in order of size: galaxy, planet, star, and comet.2. What force is responsible for holding galaxies together?3. A galaxy is 100 000 light years from Earth. When we look at the galaxy through a telescope, we are

seeing it as it was 100 000 years ago. Explain why.4. If the nearest star is 4 light years away, show it would take a rocket travelling at 11 km/s (the speed

needed to just escape the Earth) about 109 000 years to get there. (Speed of light = 3 × 108 m/s.)5. Suggest why astronomers find it so difficult to detect planets around stars other than the Sun.

The Universe consists of:Planets, moons, asteroids, and comets.

About 13 000 km

Which orbit . . .

SpiralBarredspiral

Elliptical

Galaxies can have many differentshapes.

Band ofstars seen

Stars which also come in avariety of forms (main sequence,

red giants, and white dwarves, see p122).

Which form themselves into clustersof billions of stars called galaxies.

Rotates veryslowly, once every240 million years.

The nearest star to Earth is about4.2 light years away. Since the light

has taken 4.2 years to reach Earth wesee the star as it was 4.2 years ago.

Each galaxy is separated from itsneighbours by many times it own diameterso, as the Universe contains billions ofgalaxies, it is unimaginably vast.

Astronomers have detectedplanets around some nearby stars.

If even a small proportion of thestars in the rest of the Universe

have planets, some astronomersthink it is very likely that there is

life elsewhere in the Universe.

Like the solar system, galaxies areheld together by gravity.

The Universe ismade up of billions

of galaxies.The further we look into space the

longer the light has been travelling sothe further back in time we are seeing.

Some astronomers are looking for signals sent by intelligent life from elsewhere inthe Universe. This is called the ‘search for extraterrestrial intelligence’ or SETI.

A light year is thedistance light will

travel (at 3 × 108 m/s)in a year. It is about

9.5 × 1015 m or9.5 million, million

kilometres.

Galaxiesmay alsocontainclouds ofdust and gascallednebulaeandblackholes.

100 000 light years

2/3

1/3

1.4 million km Our Sun is in an arm of a spiral galaxy wecall the Milky Way. We see this as a band ofstars across the night sky as we look throughthe plane of our galaxy.

COOL

HOT

SUN

OUR PLACE IN THE UNIVERSE The SunFor many years, scientists could not work out the source of energy for the Sun. Some thought the energy wasreleased as the Sun shrank in size releasing gravitational potential energy. Others thought it was a chemicalreaction like coal burning in a fire. However, geologists knew that the age of the Earth was about 5000 millionyears old and none of these ideas would provide enough energy to keep the Sun’s energy output at the observedrate for anything like that long.

We now know that the Sun is about 4600 millionyears old and its energy comes from nuclear fusion

(see p78 for more details). In the core, underextreme pressure and temperature, hydrogen nucleiare forced together to form helium nuclei releasing

vast amounts of energy. There is enough fuel foranother 5000 million years.

Einstein’s famous relation ∆E =∆mc2, shows thisenormous energy release, ∆E, comes at the expenseof a small overall loss in the mass of the particles,∆m, linked by the speed of light c = 3 × 108 m/s.

Inside the Sun, 600 million tonnes of hydrogen areconverted in nuclear fusion reactions every second,and 4 million tonnes of this is converted into energy.

Questions1. What provides the energy for the Sun?2. How can you guess the temperature of a star simply by looking at it? (N.B. Never look directly at the Sun.)3. What problems can solar flares cause on Earth?4. If the Sun looses 4 million tonnes (4 × 109 kg) every second, use ∆E =∆m × (3 × 108 m/s)2 to calculate the

energy output of the Sun per second, i.e. its power. 5. Show that if the Sun has a diameter 109× that of the Earth, its volume is over 1 million times greater.

Mostly hydrogen,with some heliumand a few otherelements.

Convective zone, where energyis transported to the surface

by convection. Temperature isabout two million ºC.

Solar flares are clouds ofcharged particles and

electromagnetic radiationejected at high speed. Thesetravel across space and may

collide with the Earth,distorting its magnetic field.

This can cause large induced currents in

power lines, resulting in

blackouts.They can also interfere with

communications,or damage satellites.

Rotation period 25.4 days, longer at

the poles, because theSun is not a solid body.

The colour of a stardepends on itstemperature.

RED BLUE

Diameter 1392 000 km, 109 timesthat of Earth.

Mass is only 333 000 times greater.

Light can exert a force on particles of matter. We do not usually notice this because the force is

extremely small. However, with the amount oflight in the Sun the pressure is significant andstops the outer layers collapsing under gravity.

Lower atmosphere, the chromosphere –temperature rises

again from 6000°C to50 000°C.

Outer atmosphere, thecorona. Very hot, about

one million ºC.

Photosphere, where energyis radiated into space –temperature is about

6000°C.

Core,

wherefusion takes

place.14 000 000°C.

20 000 km

120

e–

121

Questions1. What piece of apparatus could be used to split starlight into its spectrum?2. Explain how scientists can estimate the temperature of a star by analyzing the light received on Earth

from it.3. Why are there black lines present in the spectra of stars? What information does the position of the lines

give us?4. Here are the line spectra of two stars together with the line spectra of some common elements measured

in a laboratory. Which elements are present in each star?

OUR PLACE IN THE UNIVERSE Stars and Their Spectra

1. More advanced physics tellsus the electrons inRutherford’s model of theatom should lose energy andspiral into the nucleus.

By splitting the starlightup into its spectrum.

2. Clearly, this does not happen,atoms are stable. Neils Bohrsuggested that the electronscould only have certain energy levels, like the rungs on a ladder.

All hot bodies, like stars, producea continuous spectrum ofwavelengths.

Intensity (brightness)Hot

Medium

Cool Temperature isalso related to

brightness.

The line spectrum identifies whatelements are present in the outer

layers of the star.

Continuous spectrum

(See p43)

3. If an electron is given enough energy, it escapescompletely. The atom has been ionized.

4. To move up levels electrons can absorb light energy. Only a precise wavelength, related to the energy difference between levels will be absorbed.

5. Moving down levels releases light of a specific wavelength that depends on theenergy difference between levels. Minimum energy; electronscannot go lower and so cannot crash into the nucleus.

Energy level; electrons can only have certain energies in the atom, not any energy in between levels, like a person standing on a ladder can only stand on the rungs, not in

between.

Cool outer

layerAtoms in the

outer layer ofthe star absorblight of certain

wavelengthsaccording to

the energy levels in theatoms present.

Hotcore

Specificwavelengths not present = black.

Star

The wavelength (colour) of the most intense light can be related totemperature.Cooler stars → redderHotter stars → bluer

Wavelength

Brightness can be used to find the distance tothe star (by the inverse square law [see p31]).

This is an absorption line spectrum as certain wavelengthshave been absorbed by the outer layers of the star.

The missing colours are unique to theatoms that absorbed them. Each elementproduces its own unique line spectrum.

Absorption spectra can be thought of as like a fingerprint, orbarcode, uniquely identifying the element responsible.

Starlight fromtelescope

Analyzing the light from stars can tell us . . .

• How bright they are (and thereforehow far away they are).

• What elements they contain. • How fast they are moving.

Every element has a unique

set of energy levels and absorbs

a unique set of wavelengths.

Spectrum

Each colouris a uniquewavelength.

a.

b.

Calcium Hydrogen

Iron Magnesium

122

OUR PLACE IN THE UNIVERSE The Life Story of a Star

This massive explosionreleases so much energy thatthe outer layers are blown off

and their nuclei are rippedapart and reformed in every

possible way, making thewhole range of elements

in the periodic table.

FusionNuclei are moving so fast

that when they collide theirelectrostatic repulsion

cannot keep them apart andthey join.

Vastamounts of

energyreleased.

5. The nebulabecomes aprotostar.

6. The nebula beginsto heat up.

3. Other moleculesare gravitationallyattracted to thisdenser region.

4. The molecules moving towardsthis region gain kinetic energy(like a ball falling to Earth).Therefore, as the nebula contractsthe gas molecules gain kineticenergy. Kinetic energy of acollection of gas molecules isusually referred to as heat energy.

Another way of thinking about thisis to consider that the gravitationalattraction is compressing the gas,and from the gas laws (p66) thetemperature rises proportionally tothe pressure.

It is made mainly of hydrogen gas, but there may besmall amounts of otherelements and dust.

A cloud of gas inspace is called anebula.

Black holessometimes attractmatter from aneighbouringstar. This spiralsinto the blackhole, like waterdown a plughole.This releases X-rays thatastronomers canuse to detecttheirpresence.

This is a region of space where,because a very large mass hasbeen compressed into a tiny

(possibly zero) volume, gravity isso strong not even light can travel

fast enough to escape from it.

Supernova

Briefly as brightas a wholegalaxy.

The matter here is incredibly dense; 1 cm3 has a mass of about 10 000 000 tonnes.Neutron star

Blackhole

X-rays

Billions of years

The

debriscan

forma

nebula, fromw

hichIf

them

assof

theneutron

staris large enough

The remaining core is formed entirely

Temperature

1. The gas tends to bequite cool, preventing itdispersing into space.

2. Sometimes thedensity in partsof the nebulaincreases.

rise to 10

newstars,like

ourSun

, may

form.

of neutrons. It is a neutron star.

evenit

will collapse

undergravity

to

forma black hole.

Eventually

can be

implodes

123

Questions1. What is an interstellar gas cloud called?2. Outline the history of the Sun from its formation to its current state.3. What process provides the energy to make stars shine and stop them collapsing under gravity?4. Outline what will happen to the Sun when it runs out of hydrogen fuel in its core.5. What type of star will end in supernova and what might happen to the debris from this explosion?6. What is a black hole? Can we see them?7. The early universe only contained hydrogen. Where did all the other elements we see around us come

from?

9. The energy released is enormous andthis provides the heat and light outputof the star. The escaping radiation fromthe nuclear reaction produces anoutward pressure that balances theinward gravitational attraction.

Outer layers expand and cool. Fusion ceasesand corecollapses

undergravity.

Redgiant8. Hydrogen nuclei

(protons) in the core arefused into helium nuclei.

The star remains stable untilhydrogen fuel runs out. Two possibilities.

Red supergiant

Initially the sameprocesses as in a

red giant.

no further nuclear energy

released and the core

under gravity

continues to

million °C

.

Mainsequence

star

7. Eventually the core of the starcontaining mainly hydrogen getsso hot that all the molecules fallapart into atoms, which losetheir electrons to form plasma(a mixture of free nuclei and electrons).

The sun is currently abouthalf way through its mainsequence; it is 5000 millionyears old and has another5000 million years beforeits fuel runs out.

Star fuses helium into a range ofheavier elements as far as iron.

Whitedwarfstar

Blackdwarf

The remaining core,mainly made ofcarbon, cannot

undergo any furthernuclear reactions,

but is still white hot.

This cools tobecome a

‘Ave

rage

’ siz

ed s

tars

,

like

the

Sun.

Coreheats

upfurther

asthe

pressurerises

Mas

sive

star

s,ty

pica

llyab

out

Eventually the core gets hot enough tofuse helium into heavier elements and

the resulting pressure from the nuclear reaction jettisons

a number of shells of gas from the outer layer

into space to form a planetary nebula.

10×

size

ofSu

n.

Mas

sive

star

sco

nsum

eth

eir

fuel

muc

hfa

ster

soha

vesh

orte

rliv

es.

Average stars –billions of years.

Large stars –millions of years.

Mars

124

OUR PLACE IN THE UNIVERSE How Did the Solar System Form?

Questions1. What force is responsible for keeping the planets in orbit around the Sun?2. Explain why the rocky planets are found close to the Sun, whilst the gas planets are found further away.3. What evidence is there that the planets formed by collisions between lumps of dust and rock?4. The explanation of how the solar system formed is just a theory. Suggest why scientists have found it

difficult to get evidence to support the theory.

We saw on p122 that the Sun began to form when a nebula (of gas and dust) collapsed under gravity. Thecentre of the nebula began to heat up until about 4500 million years ago, when the temperature was highenough, fusion started, and the Sun became a star.

1. Orbiting around the newly formedSun was the remains of the gas anddust from the nebula.

There is evidence forradiation pressure because

comets tails, formed from gaslike the gas in the early solar

system, always point awayfrom the

Sun as the gas is blown away

by the radiation from the Sun.

Gas

Radiation pressure

Massive dustparticles

Light gasparticles

2. The debris’ gravitational attraction to theSun kept it in orbit, but the pressure fromthe radiation escaping from the Sunpushed the lighter gases into a largerorbit, leaving more massive dust particlesorbiting closer to the Sun.

And so on The heat generated by these collisions melted the rocks allowing the young planets

to form into spheres before they cooled down.

Craters on other planets areevidence for these collisions,which still continue. We callthe small rocks asteroids. Plate tectonics covers up craters on Earth, but there are still some impact craters to be seen. Scientists think these may explain some extinction processes such as that of the dinosaurs.

Ice / rock

Jupiter Saturn Uranus Neptune Pluto

Pluto does not fit this pattern. It is suggestedthat it has been captured by the Sun’s gravity

and did not form in the solar system.

The gases further out in the solar system alsocollected together to form the gas planets.

4. The dust particles collided witheach other and began to collectinto larger clumps. These grewas they collected more dust intorocks, which eventually joinedto form planets.

Asteroidbelt

The strong gravitational fieldof Jupiter prevents themcollecting into a planet.

Some astronomers think the asteroid belt,between Mars and Jupiter, might be theremains of planets that collided, perhapsdue to the influence of Jupiter’s verystrong gravitational field.

3.

Sun

Sun

Jupiter

126

Questions1. What is the Doppler effect? Suggest where you might be able to observe the Doppler effect in everyday

life.2. If a galaxy was moving towards the Earth, how would the light received from it be affected? What if it

was moving away?3. Explain what led Hubble to propose that the Universe is expanding.4. What was the Big Bang? Suggest two pieces of evidence for this theory of the Universe.5. Outline three possible fates for the Universe. What two factors will dictate which outcome actually occurs?6. Suggest some reasons why scientists are uncertain about the age and the fate of the Universe.7. Make a list of three controversial facts in this topic. Explain why they are controversial. If possible,

suggest some data scientists could collect to try to settle the dispute.

OUR PLACE IN THE UNIVERSE The Expanding Universe – Further Evidence

Big BangHot and dense

Lots of high energy light

Expansion and cooling

NowLight from Big Bang has

lost energy and beenstretched to microwaves.

Space is full of microwaves.

About 10% of the ‘snow’on an untuned TV.

This Cosmic microwave

background is good evidence for

the Big Bang Theory.

As gravity acts between all masses, it slowsdown the expansion of the Universe. Just like

a ball thrown upwards slows down. Possible fates forthe Universe

1. Universeexpands forever(Universe has asmall mass).

2. Universe stopsexpanding.

3. Universe re-collapsesunder gravitationalattraction (Universe

has a large mass).

‘BigCrunch’

Time

Rates of expansioncorresponding to differentoutcomes are all similarhere – hard to tell which

outcome will occur.Now

Size of Universe

The age of the Universe can be calculatedby measuring how far galaxies havemoved and their speed.

The calculated age assumes that theUniverse has expanded at a steady rate.

If the expansion is slowing due to gravity, the actual age is less than this.

Our best estimate of the age of theUniverse is

14 thousand millionyears

Some scientists disagreebecause they think somegalaxies are older than this.

Other theories

Steady State

Universeexpands

New mattergenerated in thespace created.

Universe continuallyexpanding and contracting.

Which of fates 1, 2, or 3 will occurdepends on the total mass of theUniverse and the rate of expansion.

We need to calculate the mass ofthe Universe.

But we think the majority of themass of the Universe is ‘darkmatter’, which we cannot see.

Measuring this and the exact rateof expansion is very hard so theage and fate of the Universeremain very controversial.

Size ofUniverse

Time

Big Crunchfollowed byanother BigBang.

Oscillating Universe